The etching of surfaces by ion bombardment from DC or RF plasmas is a widely used technique. The purpose of the technique can range from cleaning applications, where a few atom layers are removed from the target surface, to applications where many micrometers are removed to obtain a desired etch. In conjunction with this process it is also possible, and in fact is done, to analyze the elements being removed from the surface using a mass spectrometer to obtain a depth profile. Initially, it was from this particular application that the present invention was developed.
A plasma etching and analyzing device for performing the processes mentioned above has been made and sold by Leybold-Heraeus Vacuum Products Inc. and designated the INA3 Secondary Neutrals Mass Spectrometer (SNMS). The construction and general operation of the INA3 system are described by N. Kelly and U. Kaiser in "SNMS Produces SIMS Quality Without Matrix Effects", Research & Development, August 1987, pages 58-61, Cahners Publishing Company. Use and results of use of the INA3 device are described in the literature such as by A. Wucher et al in "Relative Elemental Sensitivity Factors in Secondary Neural Mass Spectrometry", J. Vac. Sci. Technol. A, Vol.6, No. 4, July/August 1988, pages 2265-2270 and in other articles in that same issue. The INA3 SNMS device employs an RF coupled plasma developed in a low pressure Ar gas environment. A sample is placed behind an aperture into the plasma chamber. By placing an appropriate negative bias voltage onto the sample, Ar ions are drawn from the plasma onto the sample surface, and neutral atoms are ejected from the sample surface, postionized in the plasma and then analyzed by a quadropole mass spectrometer.
The measurement with SNMS requires that the fluence of sputtering ions be very uniform across a large area of the sample surface (about 20 mm.sup.2), and the capability for lateral uniform sputtering over such a large area, combined with reasonable detection limits has been demonstrated for the INA3 instrument. The problem arises in achieving this capability on a daily basis, as required for routine applications. Variations in instrument parameters, including sample positioning and evolution of plasma conditions, all influence the lateral uniformity of the bombarding ions, making routine and systematic application of the technique difficult.